Problem: Which of these numbers is prime? ${3,\ 18,\ 65,\ 93,\ 99}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 18 are 1, 2, 3, 6, 9, and 18. The factors of 65 are 1, 5, 13, and 65. The factors of 93 are 1, 3, 31, and 93. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 3 is a prime number.